Implantable pulse generators using rechargeable zero-volt technology lithium-ion batteries

ABSTRACT

An implantable medical device, such as an implantable pulse generator (IPG) used with a spinal cord stimulation (SCS) system, includes a rechargeable lithiumion battery having an anode electrode with a substrate made substantially from titanium. Such battery construction allows the rechargeable battery to be discharged down to zero volts without damage to the battery. The implantable medical device includes battery charging and protection circuitry that controls the charging of the battery so as to assure its reliable and safe operation. A multi-rate charge algorithm is employed that minimizes charging time while ensuring the battery cell is safely charged. Fast charging occurs at safer lower battery voltages (e.g., battery voltage above about 2.5 V), and slower charging occurs when the battery nears full charge higher battery voltages (e.g., above about 4.0 V). When potentially less-than-safe very low voltages are encountered (e.g., less than 2.5 V), then very slow (trickle) charging occurs to bring the battery voltage back up to the safer voltage levels where more rapid charging can safely occur. The battery charging and protection circuitry also continuously monitors the battery voltage and current. If the battery operates outside of a predetermined range of voltage or current, the battery protection circuitry disconnects the battery from the particular fault, i.e. charging circuitry or load circuits.

[0001] This application is a divisional of U.S. application Ser. No.09/627,803, filed Jul. 28, 2000, which claims the benefit of U.S.Provisional Application Serial No. 60/146,571, filed Jul. 30, 1999,which provisional application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to implantable pulsegenerators, e.g., a pulse generator used within a Spinal CordStimulation (SCS) system or other type of neural stimulation system.More particularly, the present invention relates to the use of arechargeable zero-volt technology lithium-ion battery within such animplantable pulse generator.

[0003] Implantable pulse generators (IPG) are devices that generateelectrical stimuli to body nerves and tissues for the therapy of variousbiological disorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder sublaxation, etc.The present invention may find applicability in all such applications,although the description that follows will generally focus on the use ofthe invention within a spinal cord stimulation system. A spinal cordstimulation system is a programmable implantable pulse generating systemused to treat chronic pain by providing electrical stimulation pulsesfrom an electrode array placed epidurally near a patient's spine. SCSsystems consist of several components, including implantable andexternal components, surgical tools, and software. The present inventionprovides an overview an SCS system and emphasizes the use of arechargeable zero volt technology battery within such a system,including the charging system used for charging the rechargeablebattery.

[0004] Spinal cord stimulation is a well-accepted clinical method forreducing pain in certain populations of patients. SCS systems typicallyinclude an implantable pulse generator, lead wires, and electrodesconnected to the lead wires. The pulse generator delivers electricalpulses to the dorsal column fibers within the spinal cord through theelectrodes implanted along the dura of the spinal cord. The attachedlead wires exit the spinal cord and are tunneled around the torso of thepatient to a subcutaneous pocket where the pulse generator is implanted.

[0005] Spinal cord and other stimulation systems are known in the art,however, to applicants' knowledge, none teach the use of a rechargeablezero-volt technology battery within the implanted portion of the system,with accompanying charging and protection circuitry, as proposed herein.For example, in U.S. Pat. No. 3,646,940, there is disclosed animplantable electronic stimulator that provides timed sequencedelectrical impulses to a plurality of electrodes so that only oneelectrode has a voltage applied to it at any given time. Thus, theelectrical stimuli provided by the apparatus taught in the '940 patentcomprise sequential, or non-overlapping, stimuli.

[0006] In U.S. Pat. No. 3,724,467, an electrode implant is disclosed forthe neural stimulation of the spinal cord. A relatively thin andflexible strip of physiologically inert plastic is provided with aplurality of electrodes formed thereon. The electrodes are connected byleads to a RF receiver, which is also implanted and controlled by anexternal controller. The implanted RF receiver has no power storagemeans, and must be coupled to the external controller in order forneural stimulation to occur.

[0007] In U.S. Pat. No. 3,822,708, another type of electrical spinalcord stimulating device is shown. The device has five aligned electrodesthat are positioned longitudinally on the spinal cord and transverselyto the nerves entering the spinal cord. Current pulses applied to theelectrodes are said to block sensed intractable pain, while allowingpassage of other sensations. The stimulation pulses applied to theelectrodes are approximately 250 microseconds in width with a repetitionrate of from 5 to 200 pulses per second. A patient-operable switchallows the patient to change which electrodes are activated, i.e., whichelectrodes receive the current stimulus, so that the area between theactivated electrodes on the spinal cord can be adjusted, as required, tobetter block the pain. Other representative patents that show spinalcord stimulation systems or electrodes include U.S. Pat. Nos. 4,338,945;4,379,462; 5,121,754; 5,417,719 and 5,501,703.

[0008] The dominant SCS products that are presently commerciallyavailable attempt to respond to three basic requirements for suchsystems: (1) providing multiple stimulation electrodes to addressvariable stimulation parameter requirements and multiple sites ofelectrical stimulation signal delivery; (2) allowing modest to highstimulation currents for those patients who need it; and (3)incorporating an internal power source with sufficient energy storagecapacity to provide several years of reliable service to the patient.Unfortunately, not all of these features are available in any onedevice. For example, one known device has a limited battery life at onlymodest current outputs, and has only a single voltage source, and henceonly a single stimulation channel (programmable voltage regulated outputsource), which provides a single fixed pattern to up to four electrodecontacts. Another known device offers higher currents that can bedelivered to the patient, but does not have a battery, and thus requiresthe patient to wear an external power source and controller. Even then,such device still has only one voltage source, and hence only a singlestimulation channel, for delivery of the current stimulus to multipleelectrodes through a multiplexer. Yet a third known device providesmultiple channels of modest current capability, but does not have aninternal power source, and thus also forces the patient to wear anexternal power source and controller. It is thus seen that each of thesystems, or components, disclosed or described above suffers from one ormore shortcomings, e.g., no internal power storage capability, a shortoperating life, none or limited programming features, large physicalsize, the need to always wear an external power source and controller,the need to use difficult or unwieldy surgical techniques and/or tools,unreliable connections, and the like. What is clearly needed, therefore,is a spinal cord stimulation system that is superior to existing systemsby providing longer life through the use of a rechargeable battery,easier programming and more stimulating features in a smaller packagewithout compromising reliability.

[0009] Regardless of the application, all implantable pulse generatorsare active devices requiring energy for operation, either powered by animplanted battery or an external power source. It is desirable for theimplantable pulse generator to operate for extended periods of time withlittle intervention by the patient or caregiver. However, devicespowered by primary (non-rechargeable) batteries have a finite lifetimebefore the device must be surgically removed and replaced. Frequentsurgical replacement is not an acceptable alternative for many patients.If a battery is used as the energy source, it must have a large enoughstorage capacity to operate the device for a reasonable length of time.For low-power devices (less than 100 μW) such as cardiac pacemakers, aprimary battery may operate for a reasonable length of time, often up toten years. However, in many neural stimulation applications such as SCS,the power requirements are considerably greater due to higherstimulation rates, pulse widths, or stimulation thresholds. Poweringthese devices with conventional primary batteries would requireconsiderably larger capacity batteries to operate them for a reasonablelength of time, resulting in devices so large that they may be difficultto implant or, at the very least, reduce patient comfort. Therefore, inorder to maintain a device size that is conducive to implantation,improved primary batteries with significantly higher energy densitiesare needed. However, given the state of the art in battery technology,the required energy density is not achievable at the present time.

[0010] If an implanted battery is not used as the power source, then amethod is required to transcutaneously supply power to the IPG on acontinuous basis. For applications that require large amounts of powersuch as heart pumps and other heart-assist devices, an external powersource is the preferred choice. Power can be supplied to the device viaa percutaneous cable, or more preferably and less invasively, coupled tothe device through electromagnetic induction. The external power sourcecan be an AC outlet or a DC battery pack, which may be recharged orreplaced with new batteries when depleted. However, these systemsobviously require the patient to continually wear an external device topower the implanted pulse generator, which may be unacceptable for manypatients because they are often bulky and uncomfortable to wear, andnaturally, limit patient mobility.

[0011] One alternative power source is the secondary, or rechargeablebattery, where the energy in these batteries can be replenished byrecharging the batteries on a periodic basis. It is known in the art touse a rechargeable battery within an implant device. See, e.g., U.S.Pat. No. 4,082,097, entitled “Multimode Recharging System for LivingTissue Stimulators”, and applicant Carla Mann Wood's U.S. patentapplication Ser. No. 09/048,826, filed Mar. 25, 1998, entitled “Systemof Implantable Devices For Monitoring and/or Affecting Body Parameters”,now U.S. Pat. No. 6,208,894 which patent and patent application arelikewise incorporated herein by reference. The devices and methodstaught in this patent and application, however, comprise specializeddevices, e.g., microstimulators, or relate to specific applications,e.g., cardiac pacing, which impose unique requirements not applicable tomany IPG applications. Cardiac pacemakers with rechargeable batterieshave been developed in the past; see U.S. Pat. Nos. 3,454,012;3,824,129; 3,867,950; 3,888,260; and 4,014,346. However, these deviceswere met with limited success in regards to battery performance andmarket acceptance. Many of these devices were powered by nickel-cadmium(NiCd) batteries. NiCd's low volumetric energy density of 100 Wh/literprovided limited energy storage, and frequent charging was required.Also, its low nominal cell voltage of 1.2 V required many cells to bestacked in series, requiring cells to be closely matched for optimumperformance. NiCd batteries also suffered from a phenomenon called“memory effect,” which causes the cell to lose capacity if cycled atshallow discharge depths. Moreover, NiCd batteries have a highself-discharge rate, losing approximately 30% of their capacity permonth at body temperatures. Also, cycle life performance was poor, asNiCd batteries typically lasted fewer than 300 cycles. In addition,charging NiCd batteries was often problematic because the standardcharge termination method for NiCd batteries is somewhat complicated,requiring the need to detect a zero or negative voltage slope (dV/dt)and/or temperature slope (dT/dt). When NiCd batteries are overcharged,an exothermic reaction occurs: oxygen gas given off at the nickelelectrode recombines with the cadmium electrode to form cadmiumhydroxide. Cell leakage or venting can occur as a result of the pressureincrease in the cell. Furthermore, there may be disposal issues withNiCd batteries, as cadmium is highly toxic to the environment.

[0012] Newer battery technologies have been developed in recent years.The Nickel Metal-Hydride (NiMH) battery was developed to improve uponNiCd performance. NiMH batteries were first commercially introduced in1990, and are in many ways similar to NiCd batteries. The main exceptionis the replacement of the cadmium electrode with a metal-hydride alloy,resulting in more than twice the volumetric energy density (>200Wh/liter). In addition, the metal-hydride is less toxic than cadmium.However, NiMH batteries suffer from some of the same drawbacks as well,including low cell voltage (1.2 V), high self-discharge (>30% permonth), difficult charge termination, low cycle life (<300 cycles), andto a lesser extent, memory effect.

[0013] Rechargeable lithium-based batteries were first developed in the1970s using lithium metal as the active electrode material. Lithium hasgreat promise as a battery material because it is the lightest of allmetals, with high cell voltage (>3 V) and high energy density. However,lithium metal in its pure form is extremely reactive, and proved to bevery unstable as a battery electrode as employed in early designs. In1990, however, Sony Corporation introduced a safer rechargeablelithium-based battery called lithium-ion (Li-ion), which used a lithiumcomposite oxide (LiCoO₂) cathode and a lithium-intercalating graphiteanode. Lithium ions, or Li⁺, instead of lithium metal, are shuttled backand forth between the electrodes (hence the nick-name, “rocking-chair”battery). Lithium-ion is superior to other rechargeable batterychemistries, with the highest volumetric energy density (>300 Wh/liter)and gravimetric energy density (>100 Wh/kg). In addition, Lithium-ionbatteries have a high nominal voltage of 3.6 V, as well as lowself-discharge (less than 10% per month), long cycle life (>500), and nomemory effect. Charge termination for Lithium-ion batteries is alsosimpler than that of NiCd and NiMH batteries, requiring only a constantvoltage cutoff. However, Lithium-ion batteries are not as tolerant toovercharging and overdischarging. If significantly overcharged,Lithium-ion batteries may go into “thermal runaway,” a state in whichthe voltage is sufficiently high to cause the electrode/electrolyteinterface to breakdown and evolve gas, leading to self-sustainingexothermic reactions. As a result, cell leakage or venting can occur. IfLithium-ion batteries are over-discharged (<1 V), the negative electrodemay dissolve and cause plating of the electrodes. This can lead tointernal shorts within the cell, as well as possible thermal runaway.Therefore, careful monitoring of the cell voltage is paramount, andbattery protection circuitry is necessary to keep the cell in a safeoperating region.

[0014] It is known in the art to use a Lithium-ion battery in animplantable medical device, see. e.g., U.S. Pat. Nos. 5,411,537 and5,690,693. However, such disclosed use requires careful avoidance ofovercharge and overdischarging conditions, as outlined above, else theimplant battery, and hence the implant device, is rendered useless.

[0015] The most recent development in rechargeable battery technology isthe Lithium-ion polymer battery. Lithium-ion polymer batteries promisehigher energy density, lower self-discharge and longer cycle lifecompared to conventional aqueous electrolyte Lithium-ion batteries. Itschemical composition is nearly identical to that of conventionalLithium-ion batteries with the exception of a polymerized electrolyte inplace of the aqueous electrolyte. The polymer electrolyte enables thebattery to be made lighter and thinner than conventional Lithium-ionbatteries by utilizing foil packaging instead of a metal can, thusallowing it to be conformable to many form factors. Lithium-ion polymerbatteries are also theoretically safer since the polymer electrolytebehaves more benignly when overcharged, generating less heat and lowerinternal cell pressure. Unfortunately, commercial realization ofLithium-ion polymer batteries has been slow and fraught with earlyproduction problems. Only recently have the major battery manufacturers(Sony, Panasonic, Sanyo) announced plans for Lithiumion polymer batteryproduction.

[0016] What is clearly needed for neural stimulation applications is aphysically-small power source that either provides a large energyreservoir so that the device may operate over a sufficiently length oftime, or a replenishable power source that still provides sufficientenergy storage capacity to allow operation of the device over relativelylong period of time, and which then provides a convenient, easy and safeway to refill the energy reservoir, i.e., recharge the battery, so thatthe device may again operate over a relatively long period of timebefore another refilling of the reservoir (recharging of the battery) isrequired.

SUMMARY OF THE INVENTION

[0017] A spinal cord stimulation (SCS) system that uses a rechargeablebattery has been invented that is superior to existing systems. Becausephysically-small power sources suitable for implantation havingsufficient capacity to power most neural stimulation applications do notyet exist, the power source used in an neural stimulation IPG (or otherimplantable medical device application) in accordance with the teachingsof the present invention is a rechargeable battery. More particularly,the present invention is directed to the use of a rechargeablelithium-ion or lithium-ion polymer battery within an implantable medicaldevice, such as an implantable pulse generator (IPG), coupled with theuse of appropriate battery protection and battery charging circuits.

[0018] In accordance with one aspect of the invention, therefore, alithium-ion or lithium-ion polymer rechargeable battery is used incombination with appropriate battery protection and charging circuitryhoused within an implantable medical device, e.g., an IPG, of a medicalsystem, e.g., an SCS system. Such use of a rechargeable batteryadvantageously assures the safe and reliable operation of the systemover a long period of time. While a preferred embodiment of theinvention is represented and described herein by way of a spinal cordstimulation (SCS) system, it is to be emphasized that theinvention—directed to the use of a lithium-ion or lithium-ion polymerrechargeable battery in an implanted medical device, includingappropriate battery protection and battery charging circuitry—may beused within any implantable medical device.

[0019] The representative SCS system with which the lithium-ion basedrechargeable battery is employed in accordance with the presentinvention provides multiple channels, each able to produce up to 20 mAof current into a 1 Ω load. To provide adequate operating power for sucha system, the SCS system employs a rechargeable battery andcharging/protection system that allows the patient to operate the deviceindependent of external power sources or controllers. Moreover, theimplanted battery is rechargeable using non-invasive means, meaning thatthe battery can be recharged as needed when depleted by the patient withminimal inconvenience. Advantageously, the SCS system herein describedrequires only an occasional recharge, is smaller than existing implantsystems, has a life of at least 10 years at typical settings, offers asimple connection scheme for detachably connecting a lead systemthereto, and is extremely reliable.

[0020] A key element of the SCS system herein described (or other systememploying an implantable pulse generator, or “IPG”) is the use of arechargeable lithiumion or lithium-ion polymer battery. Lithium-ionbatteries offer several distinct advantages over other batterychemistries: high volumetric and gravimetric energy densities, high cellvoltage, long cycle life, simple detection of charge termination, lowtoxicity, and no memory effect.

[0021] The lithium-ion or lithium-ion polymer battery used in the SCSsystem described herein is specifically designed for implantable medicaldevices. It incorporates several distinct features compared toconventional lithium-ion batteries. The battery case is made from ahigh-resistivity Titanium alloy to reduce heating from eddy currentsinduced from the electromagnetic field produced by inductive charging.The case is also hermetically-sealed to increase cycle life and shelflife performance. Most importantly, the battery is specifically designedto allow discharge to zero volts without suffering irreversible damage,which feature is referred to herein as “zero-volt technology”. Thisfeature is significant because conventional lithium-ion batteries cannot operate at low voltages (less than about 1 V) without damageoccurring to the negative electrode. Thus, should an implant device witha conventional lithium-ion battery be operated until the implantedbattery is nearly discharged (˜2.5 V), and if the battery is notsubsequently recharged for any one of many reasons, the battery willnaturally self-discharge to below 1 V in less than six months. If thisoccurs, the performance and safety of the cell may be compromised. Incontrast, the present invention relates to an implantable electricalstimulator capable of recharging its lithiumion battery from anelectrical potential of 0 V up to normal operating voltages, e.g.,approximately 4 V. The invention takes advantage of new batterytechnology that allows discharge down to 0 V without damage to the cell.

[0022] In accordance with one aspect of the present invention, the SCSsystem utilizes a non-invasive, electromagnetic induction system tocouple the energy from an external power source to the implantedcharging circuitry for recharging the battery. The charging circuitrycontains a charge controller that converts the unregulated induced powerinto the proper charging current. The level of the charging current isdetermined by a state machine-type algorithm that monitors the voltagelevel of the battery. In one embodiment, when the battery voltage isbelow 1 V, for example, the battery is charged with a very low currentof C/50 ({fraction (1/50)} of the battery capacity) or less. When thebattery voltage surpasses 1 V, the battery is charged at a rate ofapproximately C/25. When the battery voltage surpasses 2.5 V, thebattery is charged at the maximum charge rate of approximately C/2,until the battery voltage nears its desired full-charge voltage, atwhich point the charge rate may again be reduced. That is, fast chargingoccurs at the safer lower battery voltages (e.g., voltage above about2.5 V), and slower charging occurs when the battery nears full chargehigher battery voltages (above about 4.0 V). When potentiallyless-than-safe very low voltages are encountered (e.g., less than 2.5V), then very slow (trickle) charging occurs to bring the batteryvoltage back up to the safer voltage levels where more rapid chargingcan safely occur. This multi-rate charge algorithm minimizes chargingtime while ensuring the cell is safely charged. The charging circuitryalso contains a battery protection circuit that continuously monitorsthe battery voltage and current. If the battery operates outside of apredetermined range of voltage or current, the battery protectioncircuitry disconnects the battery from the particular fault, i.e.charging circuitry or load circuits. Moreover, the charging circuitry isable to monitor the state-of-charge of the battery by measuring thevoltage of the battery, since there is good correlation between batteryvoltage and state of charge in lithium-ion batteries.

[0023] In accordance with another aspect of the invention, an externalcharging system is provided that supplies the energy to the rechargeablebattery of the IPG device. Such external charging system may take one ofseveral forms or embodiments. In one embodiment, the external charger ispowered by an alternating current (AC) power supply and is manuallycontrolled by the patient. In another embodiment, the external chargeris powered by an AC power supply and is automatically controlled by theexternal controller for the implanted device. Such embodimentnecessarily employs a suitable communication link between the externalcontroller and the external charger, the communication link comprising,e.g., a cable (hard wire connection), an infrared (IR) link, or a radiofrequency (RF) link. In another preferred embodiment, the externalcharger is itself powered by a battery, which battery may be areplaceable (primary) battery, or a rechargeable battery.

[0024] The external charger may thus assume one of several forms,ranging from a table-top AC powered device to a small portable (mobile)device that uses a primary or secondary battery to transfer energy tothe implanted device. In all instances, the electrical circuitry withinthe implanted device has final control upon the acceptance or rejectionof incoming energy. The external charging system, however, is optimallycontrolled so that its operation is terminated if the implanted devicedoes not require the external energy.

[0025] In operation, the SCS system (or other system employing an IPG)monitors the state of charge of the internal battery and controls thecharging process. Then, through a suitable communication link, the SCSsystem is able to inform the patient or clinician regarding the statusof the system, including the state of charge, and makes requests toinitiate an external charge process. In this manner, the acceptance ofenergy from the external charger is entirely under the control of theimplant circuitry, e.g., the IPG, and several layers of physical andsoftware control may be used, as desired or needed, to ensure reliableand safe operation of the charging process. The use of such arechargeable power source thus greatly extends the useful life of theSCS system, or other IPG systems. This means that once the IPG isimplanted, it can, under normal conditions, operate for many yearswithout having to be explanted.

[0026] All of the above and other features combine to provide a SCSsystem employing an IPG or similar implantable electrical stimulator (orother implantable electrical circuitry, such as an implantable sensor)having a rechargeable battery that is markedly improved over what hasheretofore been available.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The above and other aspects, features and advantages of thepresent invention will be more apparent from the following moreparticular description thereof, presented in conjunction with thefollowing drawings wherein:

[0028]FIG. 1 is a block diagram that illustrates the variousimplantable, external, and surgical components of an SCS system thatemploys an implantable pulse generator (IPG) having a rechargeablebattery in accordance with the present invention;

[0029]FIG. 2 shows various components of the SCS system of FIG. 1;

[0030]FIG. 3 is a block diagram that illustrates the main components,including a rechargeable battery, of one embodiment of an implantablepulse generator (IPG) used with the invention;

[0031]FIG. 4 is a block diagram that illustrates another embodiment ofan implantable pulse generator (IPG) that may be used with theinvention;

[0032]FIG. 5 shows a representative screen on a handheld patientprogrammer that may be used with the invention;

[0033]FIG. 6 illustrates the external components of a representativeportable charging system used by the invention;

[0034]FIG. 7A shows a block diagram of the battery charging system usedwith the invention;

[0035]FIG. 7B is a functional block diagram of the preferredmisalignment and charge complete indicators used with the invention;

[0036]FIG. 8 is a state diagram illustrating the various states that maybe assumed by the implant battery charging circuitry during operation ofthe charging system;

[0037]FIG. 9 shows a block diagram of the battery charger/protectioncircuitry utilized within the external charging station of theinvention;

[0038]FIG. 10A depicts a sectional view of a preferred structure of azero-volt technology lithium-ion battery usable with the invention; and

[0039]FIG. 10B depicts the battery voltage versus time when thelithium-Ion battery of FIG. 10A has been discharged to zero volts and isrecharged using the recharging circuitry of the present invention.

[0040] Corresponding reference characters indicate correspondingcomponents throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

[0041] The following description is of the best mode presentlycontemplated for carrying out the invention. This description is not tobe taken in a limiting sense, but is made merely for the purpose ofdescribing the general principles of the invention. The scope of theinvention should be determined with reference to the claims.

[0042] At the outset, it is noted that the present invention may be usedwith an implantable pulse generator (IPG), or similar electricalstimulator and/or electrical sensor, that may be used as a component ofnumerous different types of stimulation systems. The description thatfollows relates to use of the invention within a spinal cord stimulation(SCS) system. However, it is to be understood that the invention is notso limited. Rather, the invention may be used with any type ofimplantable electrical circuitry that could benefit from deriving itsoperating power from a rechargeable battery.

[0043] Further, while the invention is described in connection with itsuse within an SCS system, it is noted that a complete description of theSCS system is not provided herein. Rather, only those portions of theSCS system that relate directly to the present invention are disclosed.A more complete description of the SCS system may be found in U.S. Pat.No. 6,516,227, application Ser. No. 09/626,010, filed Jul. 26, 2000,which application is incorporated herein by reference.

[0044] Turning first to FIG. 1, a block diagram is shown thatillustrates the various components of an SCS system wherein theinvention may be used. These components may be subdivided into threebroad categories: (1) implantable components 10, (2) external components20, and (3) surgical components 30. As seen in FIG. 1, the implantablecomponents 10 include an implantable pulse generator (IPG) 100, anelectrode array 110, and (as needed) a lead extension 120. The extension120 is used to electrically connect the electrode array 110 to the IPG100. In a preferred embodiment, the IPG 100, described more fully belowin connection with FIG. 4 or 5, comprises a rechargeable, multichannel,16 contact, telemetry-controlled, pulse generator housed in a roundedhigh-resistivity titanium alloy case to reduce eddy current heatingduring the inductive charging process. The preferred embodiment includes16 current sources, each with a programmable amplitude such that thedevice is a current-regulated, rather than a voltage-regulated system.Four pulse timing generators are used to create 4 independent groups inwhich any of 16 electrodes can be included in a positive or negativepolarity. A connector that forms an integral part of the IPG 100 allowsthe electrode array 110 or extension 120 to be detachably secured, i.e.,electrically connected, to the IPG 100. This connector may be of thetype described in U.S. patent application Ser. No. 09/239,926, filedJan. 28, 1999, incorporated herein by reference.

[0045] The IPG 100 contains stimulating electrical circuitry(“stimulating electronics”), a power source, e.g., a rechargeablebattery, and a telemetry system. Typically, the IPG 100 is placed in asurgically-made pocket either in the abdomen, or just at the top of thebuttocks. It may, of course, also be implanted in other locations of thepatient's body. Once implanted, the IPG 100 is connected to the leadsystem, comprising the lead extension 120, if needed, and the electrodearray 110. The lead extension 120, for example, may be tunneled up tothe spinal column. Once implanted, the lead system 110 and leadextension 120 are intended to be permanent. In contrast, the IPG 100 maybe replaced when its power source fails or is no longer rechargeable.

[0046] Advantageously, the IPG 100 can provide electrical stimulationthrough a multiplicity of electrodes, e.g., sixteen electrodes, includedwithin the electrode array 110.

[0047] As seen best in FIG. 2, and as also illustrated in FIG. 1, theelectrode array 110 and its associated lead system typically interfacewith the implantable pulse generator (IPG) 100 via a lead extensionsystem 120. The electrode array 110 may also be connect to an externaltrial stimulator 140, through the use of a percutaneous lead extension132 and/or an external cable 134. The external trail stimulator 140includes the same pulse generation circuitry as does the IPG 100, and isused on a trial basis for, e.g., 7-10 days after the electrode array hasbeen implanted, prior to implantation of the IPG 100, in order to testthe effectiveness of the stimulation that is to be provided.

[0048] Still with reference to FIG. 2 and FIG. 1, the hand-heldprogrammer (HHP) 202 may be used to control the IPG 100 via a suitablenon-invasive communications link 203, e.g., an RF link. Such controlallows the IPG 100 to be turned ON or OFF, and generally allowsstimulation parameters, e.g., pulse amplitude, width, and rate, to beset within prescribed limits. The HHP may also be linked with theexternal trial stimulator 140 through another link 205′, e.g., an infrared link. Detailed programming of the IPG 100 is preferably accomplishedthrough the use of an external clinician's programmer 204 (FIG. 1) whichis coupled to the IPG 100 through the HHP 202. An external charger 208,non-invasively coupled with the IPG 100 through link 209, e.g., aninductive link, allows energy stored or otherwise made available to thecharger 208 to be coupled into the rechargeable battery housed withinthe IPG 100.

[0049] Turning next to FIG. 3, a block diagram is shown that illustratesthe main components of one embodiment of an implantable pulse generator(IPG) 100 that may be used with the invention. As seen in FIG. 3, theIPG includes a microcontroller (pC) 160 connected to memory circuitry162. The pC 160 typically comprises a microprocessor and associatedlogic circuitry, which in combination with control logic circuits 166,timer logic 168, and an oscillator and clock circuit 164, generate thenecessary control and status signals which allow the pC to control theoperation of the IPG in accordance with a selected operating program andstimulation parameters. The operating program and stimulation parametersare typically stored within the memory 162 by transmitting anappropriate modulated carrier signal through a receiving coil 170 andcharging and forward telemetry circuitry 172 from an externalprogramming unit, e.g., a handheld programmer 202 and/or a clinicianprogrammer 204, assisted as required through the use of a directionaldevice 206 (see FIG. 1). (The handheld programmer is thus considered tobe in “telecommunicative” contact with the IPG; and the clinicianprogrammer is likewise considered to be in telecommunicative contactwith the handheld programmer, and through the handheld programmer, withthe IPG.) The charging and forward telemetry circuitry 172 demodulatesthe carrier signal it receives through the coil 170 to recover theprogramming data, e.g, the operating program and/or the stimulationparameters, which programming data is then stored within the memory 162,or within other memory elements (not shown) distributed throughout theIPG 100.

[0050] The microcontroller 160 is further coupled to monitoring circuits174 via bus 173. The monitoring circuits 174 monitor the status ofvarious nodes or other points 175 throughout the IPG 100, e.g., powersupply voltages, current values, temperature, the impedance ofelectrodes attached to the various electrodes E1 . . . En, and the like.Informational data sensed through the monitoring circuit 174 may be sentto a remote location external to the IPG (e.g., a non-implantedlocation) through back telemetry circuitry 176, including a transmissioncoil 177.

[0051] The operating power for the IPG 100 is derived from arechargeable power source 180. In accordance with the teachings of thepresent invention, such rechargeable power source 180 comprises alithium-ion or lithium-ion polymer battery. The advantages of using suchbatteries have been previously discussed. The rechargeable battery 180provides an unregulated voltage to power circuits 182. The powercircuits 182, in turn, generate the various voltages 184, some of whichare regulated and some of which are not, as needed by the variouscircuits located within the IPG. A particular feature of the presentinvention is the manner in which recharging occurs, on an as-neededbasis, and wherein the power circuits 182 control the charging operationso that only energy that is needed is allowed to charge the battery,thereby preventing overcharging from occurring.

[0052] As indicated previously, the power source 180 of the IPG 100comprises a rechargeable lithium-ion or lithium-ion polymer battery.Recharging occurs inductively from an external charger (shown below inFIGS. 7 and 9) to an implant depth of approximately 2 to 3 cm. Forsafety reasons, only authorized charging devices may be used to rechargethe battery. The battery is chargeable to 80% of its capacity within twohours. Moreover, at an 80% charge, a single battery discharge is able tosupport stimulation at typical parameter settings on one channel(electrode group) for at least about three weeks; and on 4 channels forapproximately one week, after 10 years of cycling. Additionally, the IPG100 is able to monitor and telemeter the status of its rechargeablebattery 180 each time a communication link is established with theexternal patient programmer 202. Typically, a telecommunicative link isestablished, and hence battery monitoring may occur, each time aprogramming event occurs, i.e., each time the patient or medicalpersonnel change a stimulus parameter.

[0053] As described, it is thus seen that any of the n electrodes may beassigned to up to k possible groups (where k is an integer correspondingto the number of channels, and in a preferred embodiment is equal to 4).Moreover, any of the n electrodes can operate, or be included in, any ofthe k channels. The channel identifies which electrodes are selected tosynchronously source or sink current in order to create an electricfield. Amplitudes and polarities of electrodes on a channel may vary,e.g., as controlled by the patient hand held programmer 202. Externalprogramming software in the clinician programmer 204 is typically usedto set parameters including electrode polarity, amplitude, pulse rateand pulse width for the electrodes of a given channel, among otherpossible programmable features.

[0054] Hence, it is seen that each of the n programmable electrodecontacts can be programmed to have a positive (sourcing current),negative (sinking current), or off (no current) polarity in any of the kchannels. Moreover, it is seen that each of the n electrode contacts canoperate in a bipolar mode or multipolar mode, e.g., where two or moreelectrode contacts are grouped to source/sink current at the same time.Alternatively, each of the n electrode contacts can operate in amonopolar mode where, e.g., the electrode contacts associated with achannel are configured as cathodes (negative), and the case electrode,on the IPG case, is configured as an anode (positive).

[0055] Further, in the preferred embodiment, the amplitude of thecurrent pulse being sourced or sunk from a given electrode contact maybe programmed to one of several discrete current levels, e.g. ±0 to ±10mA, in steps of 0.1 mA. Also, in the preferred embodiment, the pulsewidth of the current pulses is adjustable in convenient increments. Forexample, the pulse width range is preferably at least 0 to 1milliseconds (ms) in increments of 10 microseconds (ps). Similarly, inthe preferred embodiment, the pulse rate is adjustable within acceptablelimits. For example, the pulse rate preferably spans 0-1000 Hz. Otherprogrammable features can include slow start/end ramping, burststimulation cycling (on for X time, off for Y time), and open or closedloop sensing modes.

[0056] The stimulation pulses generated by the IPG 100 are chargedbalanced. This means that the amount of positive charge associated witha given stimulus pulse must be offset with an equal and oppositenegative charge. Charge balance may be achieved through a couplingcapacitor, which provides a passive capacitor discharge that achievesthe desired charge balanced condition. Alternatively, active biphasic ormulti-phasic pulses with positive and negative phases that are balancedmay be used to achieve the needed charge balanced condition.

[0057] The type of bidirectional current sources depicted in FIG. 3 maybe realized by those of skill in the art using the teachings of U.S.Pat. No. 6,181,969, application Ser. No. 09/338,700, filed Jun. 23,1999, entitled “Programmable Current Output Stimulus Stage forImplantable Device”, which patent is incorporated herein by reference.

[0058] Advantageously, by using current sources of the type disclosed inthe referenced patent application, or equivalent, the IPG 100 is able toindividually control the n electrode contacts associated with the nelectrode nodes E1, E2, E3, . . . En. Controlling the current sourcesand switching matrix 188 using the microcontroller 160, in combinationwith the control logic 166 and timer logic 168, thereby allows eachelectrode contact to be paired or grouped with other electrode contacts,including the monopolar case electrode, in order to control thepolarity, amplitude, rate, pulse width and channel through which thecurrent stimulus pulses are provided. Other output circuits can be usedwith the invention, including voltage regulated output, multiplexedchannels, and the like.

[0059] As shown in FIG. 3, much of circuitry included within the IPG 100may be realized on a single application specific integrated circuit(ASIC) 190. This allows the overall size of the IPG 100 to be quitesmall, and readily housed within a suitable hermetically-sealed case.The IPG 100 includes n feedthroughs to allow electrical contact to beindividually made from inside of the hermetically-sealed case with the nelectrodes that form part of the lead system outside of the case. TheIPG case is preferably made from titanium and is shaped in a roundedcase, as illustrated, e.g., in FIG. 2. The rounded IPG case has amaximum circular diameter D of about 50 mm, and preferably only about 45mm (or equivalent area). The implant case has smooth curved transitionsthat minimize or eliminate edges or sharp corners. The maximum thicknessW of the case is about 10 mm. Other materials, e.g. ceramic, can be usedthat provide less shielding between the recharging coils, and thusimproving efficiency.

[0060] It is thus seen that the implant portion 10 of the SCS system ofthe present invention (see FIG. 1) includes an implantable pulsegenerator (IPG) 100 with a rechargeable battery 180 as described in FIG.4. Such IPG further includes stimulating electronics (comprisingprogrammable current sources and a switching matrix and associatedcontrol logic), and a telemetry system. Advantageously, the rechargeablebattery 180 may be recharged repeatedly as needed.

[0061] In use, the IPG 100 is placed in a surgically-made pocket eitherin the abdomen, or just at the top of the buttocks, and detachablyconnected to the lead system (comprising lead extension 120 andelectrode array 110). While the lead system is intended to be permanent,the IPG may be replaced should its power source fail, or for otherreasons. Thus, a suitable connector, e.g., the snap-on tool-lessconnector disclosed in U.S. patent application Ser. No. 09/239,926,filed Jan. 28, 1999, or other suitable connectors, may advantageously beused to make the connection between the lead system and the IPG 100.This '926 patent application is incorporated herein by reference.

[0062] Once the IPG 100 has been implanted, and the implant system 10 isin place, the system is programmed to provide a desired stimulationpattern at desired times of the day. The stimulation parameters that canbe programmed include the number of channels (defined by the selectionof electrodes with synchronized stimulation), the stimulation rate andthe stimulation pulse width. The current output from each electrode isdefined by polarity and amplitude.

[0063] The back telemetry features of the IPG 100 allow the status ofthe IPG to be checked. For example, when the external hand-heldprogrammer 202 (and/or the clinician programmer 204), initiates aprogramming session with the implant system 10 (FIG. 1), the capacity ofthe battery is telemetered so that the external programmer can calculatethe estimated time to recharge. Any changes made to the current stimulusparameters are confirmed through back telemetry, thereby assuring thatsuch changes have been correctly received and implemented within theimplant system. Moreover, upon interrogation by the external programmer,all programmable settings stored within the implant system 10 may beuploaded to one or more external programmers.

[0064] Turning next to FIG. 4, a hybrid block diagram of an alternativeembodiment of an IPG 100′ that may be used with the invention isillustrated. The IPG 100′ includes both analog and digital dies, orintegrated circuits (IC's), housed in a single hermetically-sealedrounded case having a diameter of about 45 mm and a maximum thickness ofabout 10 mm. Many of the circuits contained within the IPG 100′ areidentical or similar to the circuits contained within the IPG 100, shownin FIG. 3. The IPG 100′ includes a processor die, or chip, 160′, an RFtelemetry circuit 172′ (typically realized with discrete components), acharger coil 171′, a lithium ion or lithium ion polymer battery 180,battery charger and protection circuits 182′, memory circuits 162′(SEEROM) and 163′ (SRAM), a digital IC 191′, an analog IC 190′, and acapacitor array and header connector 192′.

[0065] The capacitor array and header connector 192′ includes 16 outputdecoupling capacitors, as well as respective feed-through connectors forconnecting one side of each decoupling capacitor through thehermetically-sealed case to a connector to which the electrode array110, or lead extension 120, may be detachably connected.

[0066] The processor 160′ is realized with an application specificintegrated circuit (ASIC) that comprises the main device for fullbidirectional communication and programming. The processor 160′ utilizesa 8086 core (the 8086 is a commercially-available microprocessoravailable from, e.g., Intel, or a low power equivalent thereof, 16kilobytes of SRAM memory, two synchronous serial interface circuits, aserial EEPROM interface, and a ROM boot loader 735. The processor die160′ further includes an efficient clock oscillator circuit 164′ and amixer and modulator/demodulator circuit implementing the QFAST RFtelemetry method supporting bi-directional telemetry at 8 Kbits/second.QFAST stands for “Quadrature Fast Acquisition Spread SpectrumTechnique”, and represents a known and viable approach for modulatingand demodulating data. The QFAST RF telemetry method is furtherdisclosed in U.S. Pat. No. 5,559,828, incorporated herein by reference.An analog-to-digital converter (A/D) circuit 734 is also resident on theprocessor 160′ to allow monitoring of various system level analogsignals, impedances, regulator status and battery voltage. In thepreferred embodiment, the A/D converter circuit 734 comprises atwelve-bit A/D converter. The processor 160′ further includes thenecessary communication links to other individual ASIC's utilized withinthe IPG 100′. The processor 160′, like all similar processors, operatesin accordance with a program that is stored within its memory circuits.

[0067] The analog IC (AIC) 190′ comprises an ASIC that functions as themain integrated circuit that performs several tasks necessary for thefunctionality of the IPG 100′, including providing power regulation,stimulus output, and impedance measurement and monitoring. Electroniccircuitry 194′ performs the impedance measurement and monitoringfunction. The main area of the analog 190′ is devoted to the currentstimulus generators 186′. These generators 186′ may be realized usingthe circuitry described in the previously-referenced patent application,or similar circuitry. These generators 186′ are designed to deliver upto 20 mA aggregate and up to 12.7 mA on a single channel in 0.1 mAsteps, which resolution requires that a seven (7) bit digital-to-analog(DAC) circuit be employed at the output current DAC 186′. Regulators forthe IPG 100′ supply the processor and the digital sequencer with avoltage of 2.7 V±10%. Digital interface circuits residing on the AIC190′ are similarly supplied with a voltage of 2.7 V±110%. A regulatorprogrammable from 5V to 18V supplies the operating voltage for theoutput current DACs 186′. The output current sources on the analog ICthus include sixteen bidirectional output current sources, eachconfigured to operate as a DAC current source. Each DAC output currentsource may source or sink current, i.e., each DAC output current sourceis bidirectional. Each DAC output current source is connected to anelectrode node. Each electrode node, in turn, is connected to a couplingcapacitor Cn. The coupling capacitors Cn and electrode nodes, as well asthe remaining circuitry on the analog IC 186′, are all housed within thehermetically sealed case of the IPG 100. A feedthrough pin, which isincluded as part of the header connector 192′, allows electricalconnection to be made between each of the coupling capacitors Cn and therespective electrodes E1, E2, E3, . . . or E16, to which the DAC outputcurrent source is associated.

[0068] The digital IC (DigIC) 191′ functions as the primary interfacebetween the processor 160′ and the AIC output circuits 186′. The mainfunction of the DigIC 191′ is to provide stimulus information to theoutput current generator register banks. The DigIC 191′ thus controlsand changes the stimulus levels and sequences when prompted by theprocessor 160′. In a preferred embodiment, the DigIC 191′ comprises adigital application specific integrated circuit (digital ASIC).

[0069] The RF circuitry 172′ includes antennas and preamplifiers thatreceive signals from the HHP 202 and provide an interface at adequatelevels for the demodulation/modulation of the communication frames usedin the processor 160′. Any suitable carrier frequency may be used forsuch communications. In a preferred embodiment, the frequency of the RFcarrier signal used for such communications is 262.144 KHz, orapproximately 262 KHz. A transmitter section receives digital transmitsignals from the quadrature components, T×I and T×Q, of the data asgenerated on the 262 KHz carrier. The T×I and T×Q signals are coupleddirectly into the antenna during transmit. Additionally, the transmitsection couples the antenna to the receiver during a receive mode. Thetransmitter section is responsible for antenna tunning and couplingwhile minimizing the processor noise to the RF signal. Appendix Bcontains additional information regarding the RF communications thatoccur between the IPG and external devices, e.g., the handheldprogrammer 202.

[0070] A receiver portion of the RF circuitry 172′ receives an incomingRF signal through a coupling circuit, amplifies the signal, and deliversit to a mixer located inside of the processor 160′.

[0071] The RF circuitry 172′ also includes an antenna. The antenna, in apreferred embodiment, comprises a ferrite rod located in an epoxy headerof the IPG case. The antenna makes electrical connection to the IPGcircuitry via two feedthrough pins included within the header connector192′ (the other pins providing electrical connection to the individualelectrodes located in the electrode array 110).

[0072] The Battery Charger and Protection Circuits 182′ provide batterycharging and protection functions for the Lithium Ion battery 180. Acharger coil 171′ inductively (i.e., electromagnetically) receives rfenergy from the external charging station. The battery 180 preferablyhas a 720 mWHr capacity. The preferred battery 180 has a life of 500cycles over 10 years with no more than 80% loss in capacity. The batterycharger circuits perform three main functions: (1) during normaloperation, they continually monitor the battery voltage and providecharge status information to the patient at the onset of a communicationlink, (2) they ensure that the battery is not over-discharged, and (3)they monitor the battery voltage during a charging cycle to ensure thatthe battery does not experience overcharging. These functions areexplained in more detail below in conjunction with FIGS. 7, 8 and 9.

[0073] Next, a representation of one embodiment of the HHP 202 is shownin FIG. 5. As seen in FIG. 5, the HHP includes a lighted display screen240 and a button pad 241 that includes a series of buttons 242, 243, 244and 245. (The number of buttons shown in FIG. 5 is exemplary only; anynumber of buttons may be employed.) The buttons provided within thebutton pad 241 allow the IPG to be tuned ON or OFF, provide for theadjustment or setting of up to three parameters at any given time, andprovide for the selection between channels or screens. Some functions orscreens may be accessible by pressing particular buttons in combinationor for extended periods of time. In a preferred embodiment, the screen240 is realized using a dot matrix type graphics display with 55 rowsand 128 columns.

[0074] The button pad 241, in a preferred embodiment, comprises amembrane switch with metal domes positioned over a flex circuit, whichbonds to the top housing of the HHP. A keypad connector connectsdirectly a printed circuit board (PCB) of the HHP, and the bonding tothe housing seals the connector opening.

[0075] In a preferred embodiment, the patient handheld programmer 202 isturned ON by pressing any button, and is automatically turned OFF aftera designated duration of disuse, e.g., 1 minute. One of the buttons,e.g., the IPG button 242, functions as an ON-OFF button for immediateaccess to turn the IPG on and off. When the IPG is turned ON, allchannels are turned on to their last settings. If slow start/end isenabled, the stimulation intensity is ramped up gradually when the IPG(or ETS) is first turned ON with the HHP. When the IPG is turned OFF,all channels are turned off. If slow start/end is enabled, thestimulation intensity may be ramped down gradually rather than abruptlyturned off. Another of the buttons, e.g., the SEL button 243, functionsas a “select” button that allows the handheld programmer to switchbetween screen displays and/or parameters. Up/down buttons 244 and 245provide immediate access to any of three parameters, e.g., amplitude,pulse width, and rate.

[0076] Also included on the screens shown on the display 240 of thehandheld programmer 202 are status icons or other informationaldisplays. A battery recharge countdown number 246 shows the estimatedtime left before the battery of the IPG needs to be recharged. A batterystatus icon 248 further shows or displays the estimated implant batterycapacity. This icon flashes (or otherwise changes in some fashion) inorder to alert the users when a low battery condition is sensed. Everytime the patient programmer is activated to program or turn on the IPG,the actual battery status of the implanted pulse generator (IPG) isinterrogated and retrieved by telemetry to reconcile actual versesestimated battery capacity. Other status icons 250 are provided thatdisplay the status of the patient-programmer-to-implant link and thepatient-programmer-to-clinician-programmer link.

[0077] As a safety feature, the physician may lock out or set selectableparameter ranges via the fitting station to prevent the patient fromaccessing undesirable settings (i.e., a lockout range). Typically,locked parameters are dropped from the screen display.

[0078] The main screen displayed by default upon activation of thehandheld programmer 202 shows amplitude and rate by channel, asillustrated in FIG. 5. As shown in FIG. 5, the display is for channel 1,the amplitude is 7.2 ma, and the rate is 100 pps. Thus, it is seen thatthe channel number (or abbreviated channel name as set by the clinicianprogrammer) is displayed on the screen with the parameters. Amplitude isthe preferred default selection (i.e., it is the parameter that isdisplayed when the unit is first turned ON).

[0079] Whenever a displayed parameter is changed, the settings of theIPG 100 are changed via telemetry to reflect the change. However, inorder to assure that the IPG has received the telemetry signal and madethe corresponding change without a discrepancy between the IPG and thevalue displayed, a back telemetry response must be received from the IPGbefore the screen value changes. Only the parameters that have not beenlocked out from the clinician's programming station are adjustable.Further, only those channels that have electrodes programmed forstimulation are selectable.

[0080] Turning next to FIG. 6, the external components of arepresentative portable charging system used with the invention areillustrated. The recharging system is used to transcutaneously rechargethe implant battery 180 of the IPG 100 as needed, via inductivecoupling. Recharging typically occurs at a rate of approximately C/2(current equal to one-half battery capacity). In order to recharge thebattery from a completely discharged state to 80% capacity,approximately two hours recharge time is required. Because of this time,a portable charger system is preferred. Hence, as seen in FIG. 8, a twopart system is preferred comprising a portable charger 208 and a basestation 210. The base station 210 is connected to an AC plug 211, andmay thus be easily plugged into any standard 110 VAC outlet. Theportable charger 208 includes recharging circuitry housed within ahousing 270 that may be detachably inserted into the charging port 210in order to be recharged. Thus, both the IPG 100 and the portablecharger 208 are rechargeable. The housing 270 is returned to thecharging port 210 between uses.

[0081] For the “Package B” embodiment shown in FIG. 6, a charging head272 is connected to the recharging circuitry 270 by way of a suitableflexible cable 274. When the IPG battery needs to be recharged, adisposable adhesive pouch 276, double sided adhesive, or a Velcro® stripis placed on the patient's skin, over the location where the IPG isimplanted. For patients with adhesive allergies, a flexible belt with anattachment means for the charger is provided such that the patient cansecure the charger over the implant. The charging head 272 is thensimply slid into the pouch, adhered to the adhesive, or fastened to thestrip, so that it is within 2-3 cm of the IPG 100.

[0082] In order for efficient transfer of energy to the IPG, it isimportant that the head 272 (or more particularly, the coil within thehead 272) be properly aligned with the IPG. Thus, in a preferredembodiment, a speaker generates an audio tone when the two devices arenot aligned, or misaligned. The misalignment indicator is activated bysensing a change in the charge coil voltage, which reflects a change inthe reflected impedance, as discussed in more detail below. When thecoil voltage is greater than a predetermined value, a beeping or otheraudible tone and/or visual indicator is activated. When the coil voltagedrops below this value, the beeping or tone or visual indicator turnsoff. The advantage of such a feature is that should the device move outof range of the implant (more likely with a non-adhesive attachmentmechanism), the misalignment indicator is activated resulting in abeeping sound or other recognizable indicator so that the patient isimmediately informed to readjust the position of the charging device. Amisalignment indicator may also be implemented in visual form, such as alight emitting diode (LED).

[0083] The external charging device also has a state of chargeindicator, i.e. an LED, or an audio tone, to indicate when the externalbattery is fully charged. This feature can also be included in a primarybattery operated charger, so that a new battery is required for eachcharge session. When charging the implant, the charger battery would bedepleted. A charge completion indicator is also provided such that whenthe charger battery is nearly depleted, a distinct tone is generated toalert the user. Also, back-telemetry with the IPG allows the chargingprocess to be monitored. When the implant battery is fully charged, asignal will be communicated from the implant to the charger, and adistinct audio tone will be generated to alert the user.

[0084] An alternative embodiment of the portable charger 208 includesthe recharging circuitry and battery and charging head housed within asingle round or oval shaped package 272, as also shown in FIG. 6(labeled “Package A”). Such package is approximately three inches indiameter and is comfortable to hold against the skin. The adhesive pouch276 need not necessarily comprise a pouch, but may utilize any suitablemeans for holding the head (coil) of the charger 208 in proper alignmentwith the IPG, such as Velcro® strips or adhesive patches.

[0085] Turning next to FIG. 7A, a block diagram of the rechargingelements of the invention is illustrated. As shown in FIG. 7A, (and asalso evident in FIGS. 3 and 4), the IPG 100 is implanted under thepatient's skin 278. The IPG includes a replenishable power source 180,such as a rechargeable battery. It is this replenishable power sourcethat must be replenished or recharged on a regular basis, or as needed,so that the IPG 100 can carry out its intended function. To that end,the recharging system of the present invention uses the portableexternal charger 208 to couple energy, represented in FIG. 7A by thewavy arrow 290, into the IPG's power source 180. The portable externalcharger 208, in turn, obtains the energy 290 that it couples into thepower source 180 from its own battery 277.

[0086] The battery 277 in the charger 208, in the preferred embodiment,comprises a rechargeable battery, preferably a Lithium-Ion battery or alithium-ion polymer battery. (Alternatively, the battery 277 maycomprise a replaceable battery.) When a recharge is needed, energy 293is coupled to the battery 277 via the charging base station 210 inconventional manner. The charging base station 210, in turn, receivesthe energy it couples to the battery 277 from an AC power line 211. Apower amplifier 275, included within the portable charger 208, enablesthe transfer of energy from the battery 277 to the implant power source180. Such circuitry 275 essentially comprises DC-to-AC conversioncircuitry that converts dc power from the battery 277 to an ac signalthat may be inductively coupled through a coil 279 located in theexternal charging head 272 (or within the round case 272′, see FIG. 6)with another coil 680 included within the IPG 100, as is known in theart. Upon receipt of such ac signal within the IPG 100, it is rectifiedby rectifier circuitry 682 and converted back to a dc signal which isused to replenish the power source 180 of the implant through a chargecontroller IC 684. A battery protection IC 686 controls a FET switch 688to make sure the battery 180 is charged at the proper rate, and is notovercharged. A fuse 689 also protects the battery 180 from being chargedwith too much current. The fuse 689 also protects from an excessivedischarge in the event of an external short circuit.

[0087] Thus, from FIG. 7A, it is seen that the battery charging systemconsists of external charger circuitry 208, used on an as-needed basis,and implantable circuitry contained within the IPG 100. In the charger208, the rechargeable Lithium-ion battery 277 (recharged through thebase station 210), or equivalent, provides a voltage source for thepower amplifier 275 to drive the primary coil 279 at a resonantfrequency. The secondary coil 680, in the IPG 100, is tuned to the sameresonant frequency, and the induced AC voltage is converted to a DCvoltage by rectifier circuit 682. In a preferred embodiment, therectifier circuit 682 comprises a bridge rectifier circuit. The chargecontroller IC 684 coverts the induced power into the proper chargecurrent and voltage for the battery. The battery protection IC 686, withits FET switch 688, is in series with the charge controller 684, andkeeps the battery within safe operating limits. Should an overvoltage,undervoltage, or short-circuit condition be detected, the battery 180 isdisconnected from the fault. The fuse 689 in series with the battery 180provides additional overcurrent protection. Charge completion detectionis achieved by a backtelemetry transmitter 690, which transmittermodulates the secondary load by changing the full-wave rectifier into ahalf-wave rectifier/voltage clamp. This modulation is, in turn, sensedin the charger 208 as a change in the coil voltage due to the change inthe reflected impedance. When detected, an audible alarm is generatedthrough a back telemetry receiver 692 and speaker 693. Reflectedimpedance due to secondary loading is also used to indicate charger/IPGalignment, as explained in more detail below in conjunction with thedescription of FIG. 9.

[0088] In a preferred embodiment, and still with reference to FIG. 7A,the charge coil 680 comprises a 36 turn, single layer, 30 AWG copperair-core coil, and has a typical inductance of 45 μH and a DC resistanceof about 1.15 ohms. The coil 680 is tuned for resonance at 80 KHz with aparallel capacitor. The rectifier 682 comprises a full-wave (bridge)rectifier consisting of four Schottky diodes. The charge controller IC684 comprises an off-the-shelf, linear regulation battery charger ICavailable from Linear Technology as part number LTC1731-4.1. Suchcharger is configured to regulate the battery voltage to 4.1 VDC. Whenthe induced DC voltage is greater than 4.1 VDC (plus a 54 mV dropoutvoltage), the charge controller 684 outputs a fixed constant current ofup to 80 mA, followed by a constant voltage of 4.1±0.05 V. Ifinsufficient power is received for charging at the maximum rate of 80mA, the charge controller 684 reduces the charge current so thatcharging can continue. Should the battery voltage fall below 2.5 V, thebattery is trickled charged at 10 mA. The charge controller 684 iscapable of recharging a battery that has been completely discharged tozero volts. When the charge current drops to 10% of the full-scalecharge current, or 8 mA, during the constant voltage phase, an outputflag is set to signal that charging has completed. This flag is used togate the oscillator output for modulating the rectifier configuration(full-wave to half-wave), which change in rectifier configuration issensed by the external charging circuit to indicate charge completion.

[0089] The battery protection IC 686, in the preferred embodiment,comprises an off-the-shelf IC available from Motorola as part numberMC33349N-3R1. This IC monitors the voltage and current of the implantbattery 180 to ensure safe operation. Should the battery voltage riseabove a safe maximum voltage, then the battery protection IC 686 opensthe charge-enabling FET switches 688 to prevent further charging. Shouldthe battery voltage drop below a safe minimum voltage, or should thecharging current exceed a safe maximum charging current, the batteryprotection IC 686 prevents further discharge of the battery by turningoff the charge-enabling FET switches 688. In addition, as an additionalsafeguard, the fuse 689 disconnects the battery 180 if the batterycharging current exceeds 500 mA for at least one second.

[0090] In a preferred embodiment, the charge-enabling FET switches 688comprise a Charge FET and a Discharge FET connected in series similar tothe FETs 701 and 701 shown in FIG. 9. In the event of a sensedmalfunction, the protection IC 686 switches off the battery 180 byturning off one of the two FET switches 688. If the battery voltage isgreater than a predetermined value, the Charge FET is turned off toblock further charging. Conversely, if the battery voltage is less thana predetermined value, the Discharge FET turns off to block furtherdischarging. If the battery voltage is zero, both the Charge FET and theDischarge FET turn off since the Battery Protection circuit is powereddirectly from the battery. Only when a charge voltage greater than VGS,or the gate-source threshold voltage, of the Charge FET is applied(which is approximately 1.5 volts) will charging resume at a rate lowerthan normal charging, i.e., 10% fo the normal rate, until the voltagehas reached a predetermined level.

[0091] Turning next to FIG. 7B, a summary of the preferred misalignmentdetection circuitry and charge completion detection circuitry used bythe invention is illustrated. As seen in FIG. 7B, an external powersource 277, e.g., a rechargeable or replaceable battery, drives a classE oscillator 275, which applies an ac signal, e.g., a signal of about 80KHz, to the primary coil 279, which coil is also labeled L1 in FIG. 7B.The voltage VR at the coil L1 is monitored by a first voltage detectioncircuit 695 and by a second voltage detection circuit 697. Both of thevoltage detection circuits 695 and 697 are connected to a speaker 693,or equivalent audible-tone generator.

[0092] The coil L1 couples energy 290 through the skin 278 of the userto an implanted coil L2 (also referred to as coil 680) that is part ofthe implanted device. The coil L2 inductively (electromagnetically)receives the ac signal 290. That is, the ac signal 290 is induced in thecoil L2 as a result of the alternating magnetic field that is createdwhen the signal is applied to the external coil L1. The alternatingsignal received at coil L2 is rectified by the switch regulator 682,thereby creating a dc voltage that is applied to charging and protectioncircuitry 685 for delivery to the implanted rechargeable battery 180.The battery 180, in turn, provides operating power for the electroniccircuitry 687 included within the implant device so that such device maycarry out its intended function, e.g., provide stimulation pulsesthrough implanted electrodes to desired nerves or body tissue. Thecircuitry 685 controls how much charging current is applied to thebattery 180 and monitors the battery voltage. The switch regulator 682operates as either a full-wave rectifier circuit or a half-waverectifier circuit as controlled by a control signal SW1 generated by thecharging and protection circuitry 685.

[0093] In normal operation, that is, when the battery 180 has beendepleted and is receiving a charge, the switch rectifier 682 operates asa full-wave rectifier circuit. During this time, assuming that the coilsL1 and L2 are properly aligned, the voltage VR sensed by voltagedetector 695 is at a minimum level because a maximum energy transfer istaking place. Should the coils L1 and L2 become misaligned, then lessthan a maximum energy transfer occurs, and the voltage VR monitored bydetection circuit 695 significantly increases. If the voltage VR isgreater than a prescribed threshold level, then voltage detectioncircuit 695 causes the speaker 693 to emit a fist audible sound, whichfirst audible sound indicates a misaligned condition. As soon as thecoils L1 and L2 are placed in proper alignment, an optimum energytransfer condition is established, causing the voltage VR to decreasebelow the threshold, thereby causing the detection circuit 695 to ceaseemitting the first audible sound. In this manner, then, it is seen thatthe detection circuit 695 operates as a misalignment detector, providingaudible feedback as to when a misaligned condition between the coils L1and L2 is present. Visual feedback could also be provided, if desired.

[0094] As the battery 180 continues to be charged, the charging andprotection circuitry continue to monitor the charge current and batteryvoltage. When the charge current and battery voltage reach prescribedlevels, which prescribed levels are indicative of a fully chargedbattery, the signal SW1 is generated by the charging and protectioncircuitry 685. The signal SW1, in turn, causes the switch rectifiercircuit 682 to switch to half-wave rectifier operation. When this changeoccurs, the voltage VR sensed by voltage detector 697 suddenly changesfrom a minimal peak-to-peak amplitude to a larger peak-to-peakamplitude, as shown in FIG. 7B. The detector 697 is adapted to sensethis sudden transient or pulsed change in amplitude, and in responsethereto causes a 2nd audible sound, e.g., a beeping sound, to begenerated through the speaker 693. This second audible sound thussignals the user that the battery is fully charged. Visual feedbackcould be used in lieu of, or in addition to, the 2nd audible sound, ifdesired.

[0095] It is noted that the operation of the misalignment andfull-charge detection circuits, illustrated in FIG. 7B, operate withoutthe use of conventional rf backtelemetry signals being sent from theimplanted device to an external device. In practice, such conventionalbacktelemetry rf signals may be used to signal the hand held programmer(HHP), when placed in telecommunicative contact with the implanteddevice, in order to provide a status report regarding the state of thecharge of the battery 180 or other conditions within the implant device.However, it should be noted that the switch rectifier 682 may bemodulated, by switching back and forth between full-wave and half-waveconditions, in order to create data words in the modulation pattern ofthe voltage VR that may be sensed and decoded at the external device,thereby providing a means for backtelemetry communication without usingconventional rf signal generation, modulation, and transmission.

[0096] Next, with reference to FIG. 8, a state diagram that shows thevarious charging states that may occur in a preferred embodiment of theinvention relative to the implant battery 180 is shown. As seen in FIG.8, and assuming a preferred lithium-ion or lithium-ion polymer batteryis used, a normal state 710 reflects that the battery voltage andcharging current are within appropriate limits. An over voltage state712 exists when the battery voltage is greater than about 4.25 V andcontinues to exist until the battery voltage is less than about 4.05 V.An undervoltage state 714 exists when the battery voltage is less than2.5 volts. The undervoltage state 714 continues to exist until thebattery voltage is greater than 2.5 volts while charging at a prescribedtrickle charge current, e.g., 10 mA. An overcurrent (charging) state 716exists whenever the charging current exceeds 80 mA. If, while in theovercurrent (charging) state 716, the battery voltage is greater than4.25 volts, then the over voltage state 712 is entered. If, while in theovercurrent (charging) state 716, the charging current exceeds 500 mAfor more than one minute, the fuse 689 opens, and a cell disconnectstate 720 is permanently entered. An overcurrent (discharging) state 718is entered whenever the battery charging current is greater than 100 mA,and continues until the battery charging current is less than 100 mA.If, while in the overcurrent (discharging) state 718, the batteryvoltage drops below 2.5 volts, then the under voltage state 714 isentered. Also, should the battery current exceed 500 mA for more thanone minute while in the overcurrent (discharging) state 718, the fuse689 opens, and the cell disconnect state 720 is permanently entered.

[0097] Thus, it is seen that through operation of the states shown inFIG. 8, the rechargeable battery 180 is fully protected from allconditions—overvoltage, undervoltage, overcharge, and undercharge—thatmay exist and which could potentially damage the battery or shorten itsoperating life.

[0098] Turning next to FIG. 9, a block diagram of the preferredcircuitry within the external charging station 208 is shown. Thecharging station comprises a portable, non-invasive transcutaneousenergy transmission system designed to fully charge the implant batteryin under three hours (80% charge in two hours). Energy for charging theIPG battery 180 initially comes from the main supply line 211, and isconverted to 5 VDC by an AC-DC transformer 694, which 5 VDC proves theproper supply voltage for the charger base station 210. When the charger208 is placed on the charger base station 210, the Lithium-ion battery277 in the charger is fully charged in approximately four hours. Oncethe battery 277 is fully charged, it has enough energy to fully rechargethe implant battery 180 (FIG. 7A or FIG. 7B). If the charger 208 is notused and left on the charger base station 210, the battery 277 willself-discharge at a rate of about 10% per month.

[0099] Still with reference to FIG. 9, once the voltage of the battery277 falls below a first prescribed limit, e.g., 4.1 VDC, during astandby mode, charging of the battery is automatically reinitiated. Inaddition, should the external charger battery 277 be discharged below asecond prescribed limit, e.g., 2.5 VDC, the battery 277 is trickledcharged until the voltage is above the second prescribed limit, at whichpoint normal charging resumes.

[0100] A battery protection circuit 698 monitors if an over voltage,under voltage, or overcurrent condition occurs, and disconnects thebattery, e.g, through opening at least one of the FET switches 701and/or 702, or from the fault until normal operating conditions exist.Another switch 699, e.g., a thermal fuse, will disconnect the batteryshould the charging or discharging current exceed a prescribed maximumcurrent for more than a prescribed time, e.g., 1.5 A for more than 10seconds.

[0101] The battery 277 provides a power source for the RF amplifier 275.The RF amplifier, in a preferred embodiment, comprises a class Eamplifier configured to drive a large alternating current through thecoil 279.

[0102] Still with reference to FIG. 9, an alignment detection circuit695 detects the presence of the IPG 100 through changes in the reflectedimpedance on the coil 279, as described above in connection with FIG.7B. Reflected impedance is a minium when proper alignment has beenobtained. This means that the steady-state voltage VI sensed at the coil279 is also at a minimum because maximum coupling occurs. When maximumcoupling is detected, e.g., when Vi is at a minimum, an audible orvisual alarm may sound. In a preferred embodiment, a first audible toneis generated whenever alignment is not achieved. Thus, as a chargingoperation begins, the first audible tone sounds, and the user seeks toposition the charger 208 (or at least to position the coil 279) at alocation that causes the first audible tone to cease. Similarly, acharge complete detection circuit 697 alerts the user through generationof a second audible tone (preferably an ON-OFF beeping sound) when theIPG battery 180 is fully charged. A fully charged condition is alsosensed by monitoring the reflected impedance through the coil 279. Asindicated above, a fully charged condition is signaled from the IPG byswitching the rectifier circuit 682 within the IPG from a full-waverectifier circuit to a half-wave rectifier circuit. When such rectifierswitching occurs, the voltage Vi suddenly increases (e.g., a transientor pulsed component appears in the voltage Vi) because the amount ofreflected energy suddenly increases. This sudden increase in Vi isdetected by the charge complete detection circuit 697, and once detectedcauses the second audible tone, or tone sequence, to be broadcast viathe speaker 693 in order to signal the user that the implant battery 180is fully charged.

[0103] Thus, it is seen that invention provides an implant device havinga rechargeable internal battery as well as the control system used tomonitor the battery's state of charge and circuitry to control thecharging process. The system monitors the amount of energy used by theimplant system and hence the state of charge of the battery. Throughbidirectional telemetry (forward and back telemetry) with the hand heldprogrammer 202 and/or the clinician programmer 204, the system is ableto inform the patient or clinician of the status of the system,including the state of charge, and further make requests to initiate anexternal charge process when needed. The acceptance of energy from theexternal charger is entirely under the control of the implanted system.Advantageously, both physical and software control exist to ensurereliable and safe use of the recharging system.

[0104] As indicated previously, conventional lithium-ion batteries arenot susceptible to being discharged to zero volts without sufferingirreversible damage. In contrast, the present invention uses alithium-ion or lithium-ion polymer battery that has been modified toallow a zero volt discharge condition to occur without causingirreversible damage to the battery. Such modified lithium-ion batteryhas a cell structure as depicted in FIG. 10A. As seen in FIG. 10A, thebattery cell includes an anode 736 and a cathode 732. The anode is madefrom graphite that is placed on a titanium substrate 740. The cathode ispreferably made from LiNiCoO₂ (lithium-nickel-cobalt oxide) that coatsan aluminum (Al) substrate 730. An electrolyte and separator 734separate the cathode from the anode.

[0105] The battery cell structure shown in FIG. 10A differssignificantly from conventional Lithium-ion batteries. First, such amodified lithium-ion battery utilizes an anode (current-collector)electrode having a substrate that is made from titanium instead ofcopper. This allows the cell to drop to zero volts without causingirreversible damage. Second, the cathode electrode of the battery ismade from LiNiCoO₂ instead of LiCoO2. Third, the separator used insidethe battery is made from a ceramic instead of PE. Fourth, for alithium-ion polymer battery, the electrolyte within the battery ispreferably realized using a solid polymer conductor instead ofLiPF6/EC+DEC, which is the electrolyte typically used in a lithium-ionbattery. Additionally, the battery case may be coated with ferrite tofurther minimize eddy current heating. A “zero-volt technology” batterymade in accordance with the present invention will always include atleast the first modification mentioned above, i.e., an anode electrodehaving a substrate made from titanium (or a suitable titanium alloy), asopposed to copper, because this is the change needed to allow thebattery cell voltage to drop to zero volts without causing irreversibledamage to the battery cell.

[0106] When a modified lithium-ion battery as shown in FIG. 10A isdischarged, it may discharge to zero volts without damage. When such adischarged-to-zero-volts battery is recharged, the voltage versuscurrent relationship during the charging operation, assuming thecharging circuit shown in FIG. 7A is employed, is substantially as shownin FIG. 10B. Of course, if the battery 180 has been discharged to zerovolts, then no circuitry within the implant device may operate becausethere is no operating power. However, as soon as an external charger 208begins coupling energy into the implant device, this energy is rectifiedand provides a dc operating voltage that allows the charge control IC684 and battery protection IC 686 to begin to function, directing theappropriate charge current to the discharged battery 180. As seen inFIG. 10B, in a very short time, e.g., within about 10 seconds or so, thebattery voltage will rise to about 0.3 V. From that initial startingpoint, the battery voltage gradually increases, almost in a linearmanner, until it reaches about 1.5 volts after about being charged forabout 2 minutes and 35 seconds. Thereafter, the battery continues tocharge (not shown in FIG. 10B), until the battery voltage reaches a fullcharge of about 4.1 volts after about 2 hours or so.

[0107] Thus, as described above, it is seen that through use of arechargeable internal battery 180 within the IPG 100, the SCS system andits control system are able to monitor the state of charge and controlthe charging process of the rechargeable battery 180. Throughbi-directional telemetry (forward and back telemetry) with the hand heldprogrammer 202 and/or the clinician programmer 204, the SCS system isable to inform the patient or clinician of the status of the system,including the state of charge, and further make requests to initiate anexternal charge process when needed. The acceptance of energy from theexternal charger is entirely under the control of the SCS implantedsystem. Advantageously, several layers of physical and software controlexist to ensure reliable and safe use of the recharging system.

[0108] Also, as described above, it is seen that the invention providesa rechargeable system for use with a medical implant device or systemthat is characterized by the use or inclusion of: (1) lithium-ion orlithium-ion polymer batteries; (2) lithium-ion zero volt batterytechnology; (3) mis-alignment indication at the external location,responsive to changes in reflected impedance from the implant location;(4) a rechargeable recharger; (5) adhesive and/or Velcro®/adhesivecharger attachment means; (6) over and under charge protectioncircuitry, including automatic shut-off circuitry; (7) back telemetry ofimplant battery charge level to external hand-held or clinician'sprogrammer; (8) an end of charging modulating indicator within theimplant device; (9) a battery status indicator on the external chargerdevice; and (10) single or dual rate charging circuitry, i.e., fastcharging at the safer lower battery voltages, and slower charging whenthe battery is nearer to full charge higher battery voltages.

[0109] While the invention herein disclosed has been described by meansof specific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

What is claimed is:
 1. An implantable pulse generator (IPG) system foruse with a spinal cord stimulation system, the IPG system comprising animplantable pulse generator and an external portable charger, whereinthe IPG comprises: an hermetically sealed case; electronically circuitryincluding memory circuits, housed within said hermetically sealed case,said electronic circuitry including a multiplicity of independentbidirectional output current sources, each output current source beingconnected to an electrode node; a multiplicity of coupling capacitors,each coupling capacitor being connected to a respective one of saidelectrode nodes; a header connecter attached to said sealed case, theheader connecter having a multiplicity of feedthrough pins that passtherethrough, wherein each of said multiplicity of coupling capacitorsis connected on the sealed side of said case to one of said feedthroughpins; an electrode array having a multiplicity of electrodes thereonexternal to said sealed case, wherein each electrode of the multiplicityof electrodes is detachably electrically connected to one of saidfeedthrough pins on a nonsealed side of said sealed case; wherein eachoutput current source generates an output stimulus current having aselected amplitude and polarity that, when the output current source isenabled, is directed to the electrode connected thereto through itsrespective feedthrough pin and coupling capacitor; a rechargeablelithium-ion battery adapted to operate over a wide range of voltagesranging from zero volts to a fully-charged voltage, said rechargeablebattery having an anode electrode having a substrate made from titanium,said rechargeable battery providing operating power for the electroniccircuitry; a secondary coil; a rectifier circuit; and battery chargerand protection circuitry that receives externally generated energythrough the secondary coil and rectifier circuit, and uses theexternally generated energy to charge the rechargeable lithium-ionbattery.
 2. The IPG system of claim 1 wherein the portable chargercomprises: a second rechargeable battery; a recharging base station thatrecharges the second rechargeable battery from energy obtained from lineac power; a primary coil; a power amplifier for applying ac powerderived from the second rechargeable battery to the primary coil; a backtelemetry receiver for monitoring the magnitude of the ac power at theprimary coil as applied by the power amplifier, thereby monitoringreflected impedance associated with energy magnetically coupled throughthe primary coil; and an alarm generator that generates an audible alarmsignal in response to changes sensed in the reflected impedancemonitored by the back telemetry receiver.
 3. The IPG system of claim 2wherein the back telemetry receiver comprises: alignment detectioncircuitry that detects when the primary coil is properly aligned withthe secondary coil included within the IPG for maximum power transfer;and charge complete detection circuitry that detects when the batterywithin the IPG is fully charged.
 4. The IPG system of claim 3 whereinthe alignment detection circuitry causes the alarm generator tobroadcast a first audible tone when the primary coil is misaligned withthe secondary coil, whereby the first audible tone stops being broadcastwhen the primary coil is properly aligned with the secondary coil. 5.The IPG system of claim 4 wherein the battery charger and protectioncircuitry within the IPG comprises: monitoring circuitry that monitorsthe voltage of the rechargeable battery and the charging current flowingto the rechargeable battery; and wherein the rectifier circuit isswitchable between a full-wave rectifier circuit and a half-waverectifier circuit, and wherein the rectifier circuit is switched tooperate as a full-wave rectifier circuit during charging of therechargeable battery, and wherein the rectifier circuit switches to ahalf-wave rectifier circuit when the rechargeable battery voltage andrechargeable battery charging current reach prescribed levels, whichprescribed levels indicate the rechargeable battery is fully charged,whereby modulation of the rectifier circuit between a full-waverectifier circuit and a half-wave rectifier circuit is used to indicatewhether the rechargeable battery is fully charged; and wherein a changein reflected impedance at the primary coil indicates a switching of therectifier circuit from a full-wave rectifier circuit to a half-waverectifier circuit, and hence indicates whether the rechargeable batteryis fully charged.